World wide video standards such as NTSC, PAL, and SECAM use interlaced video formats to maximize the vertical refresh rates while minimizing the required transmission bandwidth. The international digital video broadcast standards such as DVB-T and ATSC also use interlaced video formats to maximize the vertical refresh rates while minimizing the required transmission data rates. The international digital video standards such as ITU-R BT.601 and ITU-R BT.709 also define interlaced video formats such as 480i, 576i, and 1080i for transmission of digital video signals.
In an interlaced video format, depicted in FIG. 1A and FIG. 1B, an image frame 11 comprises at least two video fields 10a-10c. In each video field, e.g., 10a, a plurality of pixels 14 are arranged in scan lines 12a. The pixels 14 in one half of scan lines 12a of an image frame 11 are displayed on the screen during the first vertical scan period (i.e., a first video field 10a), while the pixels 14 in the other half of scan lines 12b, positioned halfway between those displayed during the first period, are displayed during a second vertical scan period (i.e., a second field 10b).
For example, in the NTSC video standard, a video field contains 262.5 scan lines, and 59.94 video fields are transmitted per second. A first video field begins with a full scan line of pixels, and ends with a left-hand half scan line containing the bottom of the image frame, as shown in FIG. 1C. An adjacent video field begins with pixels in a right-hand half scan line containing the top of the image frame and it ends with a full picture line of pixels. In PAL and SECAM standards, a video field contains 312.5 scan lines, and 50 video fields are transmitted per second. For interlaced video signals in PAL and SECAM (shown in FIG. 1D), the first field includes a right-hand half scan line containing the top line of the image frame, and ends with a full scan line, while the adjacent field starts with a full scan line, and ends with a left-hand half scan line that contains the bottom of the image frame. For these analog video standards, each video field contains scan lines for active video and for vertical blanking intervals.
In the digital 480i standard, a video field contains 240 scan lines, and 59.94 video fields are transmitted per second. In the digital 576i standard, a video field contains 288 scan lines, and 50 video fields are transmitted per second. For these digital video standards, each video field contains scan lines for active video only. For interlaced formats in MPEG-2 compressed video standard (shown in FIG. 1E), a video field is identified according to whether it contains the top or bottom scan line of the image frame. Top and bottom fields are displayed in the order that they are coded in an MPEG-2 data stream.
In the interlaced video format, each video field, e.g., 10b, has an associated field parity that indicates the relative vertical position among its scan lines 12b and those of its two temporally adjacent video fields, i.e., the previous 10a and subsequent 10c video fields. For a 2:1 interlaced video signal, as defined in various international video standards, each video field 10a-10c has an associated parity that is one of two possible parities, P1 and P2. In a normal situation, two temporally adjacent video fields 10a, 10b have opposite parities, so the scan lines 12a, 12b from the two video fields interlace with one other. For example, if the current video field 10b has parity P1, then both the previous 10a and subsequent 10c video fields should have parity P2, and vice versa.
Therefore, for any interlaced video signal, the two parities P1 and P2 alternate for each sequentially ordered video field. In normal operation, two field parity patterns (FPPs) associated with an interlaced video signal are possible. For example, a first FPP is represented by the following pattern:
2P1P2P1P2P1P2
while a second FPP is represented by the following pattern:
P2P1P2P1P2P1P2P1.
For purposes of this description, the two field parity patterns can be denoted as FPP1 and FPP2. Note that the definitions of the two field parity patterns are relative to one another. There is no absolute meaning for FPP1 or FPP2 to represent any one of the two possible field parity patterns for any interlaced video signal.
While using interlaced video formats can maximize the vertical refresh rates and minimize the required transmission bandwidth, visual artifacts such as line flicker and line crawling can also result. In an interlaced-to-progressive video format converter (i.e., a de-interlacer), the output of an interlaced input video signal is improved by converting the interlaced signal into a progressive (non-interlaced) format for display. Indeed, many modern display systems employing technologies newer than cathode-ray tube (CRT), such as liquid crystal display (LCD) and plasma display panel (PDP) systems, require an interlaced-to-progressive conversion before an image can be displayed.
Various methods exist for converting an interlaced signal to a progressive signal. Some techniques use simple spatial-temporal methods, e.g., line repetition (bob) and field insertion (weave). Other techniques are more complicated that use per-field and per-pixel motion-adaptive (MA) techniques and advanced motion compensated (MC) techniques. In all cases, however, in order to convert effectively an interlaced input video signal into a progressive video signal, accurate information relating to the field parity of each input video field is required.
During the transmission of an interlaced video signal in many cases, the field parity information is not explicitly transmitted with the video signal. Rather, the field parity information is typically embedded in the vertical and horizontal synchronization signals that are transmitted with the video signal. For example, in a NTSC, PAL, or SECAM analog video signal, a baseband luminance signal, a modulated chrominance signal, and vertical and horizontal synchronization signals are combined to form a composite video signal. In such cases, once the composite vertical and horizontal synchronization signal is extracted from the composite video signal, the field parity signal can be detected from the composite synchronization signal. In a YPbPr analog component video signal carrying an interlaced video format, the vertical and horizontal synchronization signals are usually mixed with the luminance (Y) signal to form the sync-on-luminance (SOY) signal. In such cases, the composite vertical and horizontal synchronization signal can be extracted from the sync-on-luminance (SOY) signal first so that the field parity signal can be detected from the composite synchronization signal.
During the transmission, reception, scan format conversion, and display of the interlaced video signal, the field parity of each interlaced video field must be correctly preserved or regenerated either explicitly through a dedicated field parity signal or implicitly through a field parity signal embedded in the vertical and horizontal synchronization signals. Nonetheless, the field parity information of the interlaced video signal often suffers errors and/or losses during this process. Moreover, the field parity information can also suffer errors or losses due to compatibility or interoperability problems among video devices designed and manufactured by different companies around the world. Furthermore, in addition to the problems described above, the field parity information of the interlaced video signal may be incorrectly assigned during production and/or mastering in the studio or broadcast stations. With the large volume of video program exchanges among different production sources of the programs, this problem is occurs more frequently than not.
Due to these serious problems, the field parity information of the interlaced video signal can be unreliable or even unavailable when the signal is received at the de-interlacer, the format converter, or other video processing functional block of a video display device. If not corrected, these errors or losses in the field parity information can cause highly objectionable visual artifacts such as saw teeth along sharp diagonal edges and horizontal stripes in vertically changing areas. For example, FIG. 2A and FIG. 2B depict images processed using the correct field parity (FIG. 2A) and the incorrect field parity (FIG. 2B), respectively. In FIG. 2A, the image is sharp and clear when the video fields are processed using the correct field parity. in contrast, the image in FIG. 2B has jagged edges (e.g., along the pier and between the boats) when the video fields are processing using the incorrect field parity. These visual artifacts are highly objectionable.